There is a hot-dip galvanized steel sheet to be used as a steel sheet having good corrosion resistance. This hot-dip galvanized steel sheet is widely used in various manufacturing industries such as in automobile, home electric appliance, and building material fields.
As a manufacturing method of the hot-dip galvanized steel sheet, a method in which a cold-rolled steel sheet or a hot-rolled steel sheet is used as a base steel sheet and the base steel sheet is passed through a continuous hot-dip galvanizing line, (which will be referred to as a CGL, hereinafter), to manufacture a hot-dip galvanized steel sheet is general. As a process of the CGL, it is general to use a total reducing furnace method in which in a cleaning section on the entrance side, the base steel sheet is subjected to alkaline spray degreasing and then is subjected to brush cleaning, and in an annealing section, is subjected to annealing in a reducing atmosphere to be dipped in a hot-dip galvanizing bath. Further, there is also a case to use a Sendzimir method in which at the stage prior to the annealing section, a non-oxidizing furnace is provided, and the surface-cleaned base steel sheet is preliminarily heated in the non-oxidizing furnace and then is subjected to reduction annealing in a reducing furnace to then be dipped into a hot-dip galvanizing bath.
For the purpose of further improving corrosion resistance of a hot-dip galvanized steel sheet manufactured by the process as described above, there has been proposed a high-corrosion-resistance hot-dip galvanized steel sheet with a hot-dip galvanizing layer having Al and Mg added thereto. For example, in Patent Document 1, there has been proposed a hot-dip Zn—Al—Mg—Si coated steel sheet. Further, in Patent Document 1, it has been proposed that to this hot-dip Zn—Al—Mg—Si coated steel sheet, one or two or more of Ca, Be, Ti, Cu, Ni, Co, Cr, and Mn is/are added, to thereby make it possible to obtain a coated steel sheet having further excellent corrosion resistance.
Further, in Patent Document 2, it has been disclosed that Ti, B, and Si are added to a hot-dip Zn—Al—Mg coated steel sheet to thereby improve its outward appearance.